For electrochemical analysis chemically modified electrodes offer distinct advantages over electrodes with an unmodified working surface. They are more selective. Therefore, surface fouling by precipitation or adsorption of ions other than those of interest is avoided by their use. Chemically modified electrodes also require the application of an overpotential in order to drive electrochemical reactions.
Synthesis of a self-doped conducting polymer, polyaniline, is disclosed in Epstein, Arthur J., "Synthesis of Self-Doped Conducting Polyaniline" Journal of the American Chemical Society, 112:2800-2801 (1990). Although doping has been recognized a enhancing the conductivity of conducting polymers, the use of a doped polymer as a surface modifier for an electrode to facilitate chemical analysis has not been heretofore recognized.
Undoped conducting polymers, on the other hand, have been used as surface modifiers of electrodes with mixed results. For example, a poly(3-methylthiophene)-coated electrode was stated to be useful for the detection of chromium (VI) at low levels in Teasdale, P., et al., "Selective Determination of Cr(VI) Oxyanions Using a Poly-3-methylthiophene-modified Electrode," Electroanalysis, 1:541-547 (1989). The detection of chromium ion was observed by preconcentrating the Cr(VI) ion within the polymer matrix. The selectivity of the polymer for Cr(VI) ion was said to be caused by the presence of the cation from the electrolyte used in the electrodeposition of the polymer onto the electrode, which may have been incorporated into the polymer matrix. The detection of the Cr(VI) was accomplished by cyclic voltammetry. The electrochemical mechanism which formed the basis for the detection of the Cr(VI) was reported to be non-Nernstian. The electrode also exhibited a degradation in performance after repeated use. Moreover, the experimental results were not reproducible from electrode to electrode.
In Lee, I., et al., "Aggregation of Bu.sub.4 NClO.sub.4 Electrolyte in the Presence of Dicyclohexano-18-crown-6 in Poly(3-methyl-thiophene) Conducting Polymer Film Electrodes. Unusual Transient Current Phenomena," J. Chem. Soc. Chem. Commun., 8:497-498 (1989) voltammograms of a poly(3-methylthiophene)-coated electrode in a Bu.sub.4 NClO.sub.4 -dicyclohexano-18-crown-6 acetonitrile solution has been reported. The obtained voltammograms were said to exhibit up and down fluctuations caused by the doping and undoping of the poly(3-methyl-thiophene) coating in the solution. Use of the electrodes to detect the presence of a specific ionic species in solution was not reported.
The preparation of iodine-doped and copper-doped poly(thiophene-2,5-diyl) are disclosed in Yamamoto, T., et al., "Preparation of Thermostable and Electric-Conducting Poly(2,5-Thienylene)," J. Polym. Sci., Polym. Let. Ed., 18:9-12 (1980) and in Czerwinski, A., et al, "The Effect of 'Water and Transition Metal Ion Doping' on the Conductivity of Poly(3-substituted Thiophene-2,5 diyls," J. Chem. Soc. Commun., 587:1158-1159 (1985). However, their suitability for performing sensitive electrochemical measurements, such as the detection of an ionic species in solution, has not been reported.
In Yano, Jr., et al., "Selective Nernstian Response of Poly(N,N-dimethylaniline))/Poly(o-chloroaniline) Dual-layer coated Electrode to Dissolved Iodide Ion," Chemistry Letters pp. 1943-1946 (1988) an electrode with a dual layer coated electrode was observed to be selective for the detection of I.sup.- ion in solution. The first polymer layer was doped in a 0.1M potassium iodide solution before the second layer was applied thereon and the second layer was observed to contribute to the selectivity of the electrode measurements.
It has now been found that ion doped organic conducting polymers such as polythienylenes and the like can be used as surface modifiers of electrode surfaces for sensitive and selective electrochemical measurements. The electronic properties of such conducting polymers can be reversibly controlled by the doping and undoping of the polymer. Therefore, the conductivity of such polymers can be turned on and off as desired.
Such electrodes are useful to detect the presence of trace amounts of ionic species in solution. The advantages of an electrode with a surface coated with doped poly(thiophene) capable of performing sensitive ion detection for potentiometric analysis has heretofore gone unrecognized.